Method for producing sodium tungstate, method for collecting tungsten, apparatus for producing sodium tungstate, and method for producing sodium tungstate aqueous solution

ABSTRACT

Provided are a method for producing sodium tungstate by supplying an oxidant made of sodium nitrate or sodium nitrite to bring a tungsten containing material and the oxidant into contact with each other in an atmosphere containing oxygen to thereby continuously produce a reaction product; a method for collecting tungsten using the method; and an apparatus for producing sodium tungstate. Also provided are a method for producing a sodium tungstate aqueous solution in which a reductant is introduced into a melt containing the above-described reaction product which is then dissolved in water; and a method for collecting tungsten using the method.

TECHNICAL FIELD

The present invention relates to a method for producing sodiumtungstate, a method for collecting tungsten, an apparatus for producingsodium tungstate, and a method for producing a sodium tungstate aqueoussolution.

BACKGROUND ART

Cemented carbide which contains tungsten carbide (WC) as a maincomponent and added with carbide such as titanium (Ti) and tantalum (Ta)using cobalt (Co), nickel (Ni) and the like as bonding metal for thepurpose of improving performance is excellent in hardness and abrasionresistance. Accordingly, such cemented carbide is suitably used for acemented carbide tool for metal processing.

In this case, the cemented carbide tool which cannot be used any moredue to chipping or abrasion and the chipped portion of the cementedcarbide tool are to be discarded as hard scraps. Furthermore, part ofcemented carbide powder produced in the process of manufacturing acemented carbide tool and grinding dust and the like produced by theprocessing using the cemented carbide tool are to be discarded as softscraps. These hard scraps and soft scraps contain a great amount oftungsten regarded as rare metal.

Thus, for example, a method for recycling a cemented carbide tool forrecovering tungsten carbide from a used cemented carbide tool and thelike is proposed in “Collection of Tungsten and the like from WasteCemented Carbide Tool” (Metal Resources Report; Vol. 38, No. 4, pp. 407to 413, November 2008) written by Yasuhiko Tenmaya issued by Rare MetalHigh Efficiency Collection System Development Project (Non-PatentDocument 1). The method for recycling the cemented carbide tooldisclosed in Non-Patent Document 1 is carried out as described below.

First, hard scraps and soft scraps of the cemented carbide tool arereacted with molten salt of sodium nitrate and then dissolved in water,to produce a sodium tungstate aqueous solution.

Then, an ion exchange method using ion-exchange resin is employed toproduce an ammonium tungstate aqueous solution from the sodium tungstateaqueous solution. Then, ammonium paratungstate (APT) is crystallized outfrom the ammonium tungstate aqueous solution.

Then, the ammonium paratungstate crystallized out as described above iscalcined, reduced and carbonized to thereby allow production of tungstencarbide.

Furthermore, for example, Japanese Patent National Publication No.11-505801 (Patent Document 1) proposes to use molten salt containing 60to 90 percent by mass of sodium hydroxide (NaOH) and 10 to 40 percent bymass of sodium sulfate (Na₂SO₄) for producing sodium tungstate byoxidizing the scraps of hard alloy and/or scraps of heavy metal in themolten salt. It also proposes that the reaction between theabove-described scraps and molten salt occurs within a rotary kiln thatis operated in a manner of batch processing and can be directly heated.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent National Publication No.    11-505801-   Non-Patent Document 1: Yasuhiko Tenmaya, Rare Metal High Efficiency    Collection System Development Project, “Collection of Tungsten and    the like from Waste Cemented Carbide Tool”, Metal Resources Report;    Vol. 38, No. 4, pp. 407 to 413, November 2008

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, the method disclosed in Non-Patent Document 1 poses a problemin safety of the operation since an extremely severe reaction occursbetween the molten salt of sodium nitrate and the hard scraps or softscraps of the cemented carbide tool, which causes difficulty incontrolling the reaction.

Furthermore, in the method disclosed in Non-Patent Document 1, when thehard scraps and soft scraps of the cemented carbide tool are reactedwith the molten salt of sodium nitrate, the metals regarded asimpurities such as vanadium and chromium contained in the hard scrapsand soft scraps of the cemented carbide tool may be contained in thesodium tungstate aqueous solution in the state where the metals are inthe form of water-soluble metal oxide ions. This causes a problem that asodium tungstate aqueous solution of high purity cannot be obtained.

Furthermore, in the method disclosed in Patent Document 1, although themelting point of sodium sulfate as molten salt serving as an oxidant isas high as 884° C. and the temperature during the reaction is also ashigh as 884° C. or more, the problem with regard to the industrialsafety can still be solved which results from a high exothermic reactionwhich is difficult to be controlled. However, in addition toconsiderable corrosion of the metal material, even when theabove-described reaction is developed under the high-temperatureenvironment of 1000° C., this reaction slowly proceeds. This causes aproblem that the reaction between the above-described scraps and theabove-described molten salt takes much time, which leads to an increasein loss of the energy.

Furthermore, in the method disclosed in Patent Document 1, the processis performed within a rotary kiln that is operated in a manner of batchprocessing and can be directly heated, which requires the raw materialsto be transferred into the rotary kiln and also the reaction product tobe transferred out of the rotary kiln for each batch processing. Inaddition, there is also a problem of decreased productivity since therotary kiln should be heated to a prescribed temperature before theabove-described reaction is started.

In view of the foregoing, an object of the present invention is toprovide a method for producing sodium tungstate that allows efficientproduction of sodium tungstate, a method for collecting tungsten usingthe method, and an apparatus for producing sodium tungstate.

Furthermore, an object of the present invention is also to provide amethod for producing a sodium tungstate aqueous solution that allowsproduction of a sodium tungstate aqueous solution of high purity, and amethod for collecting tungsten using the method.

Means for Solving the Problems

The present invention provides a method for producing sodium tungstatefrom a tungsten containing material. The method includes the steps ofcontinuously or intermittently supplying at least one of the tungstencontaining material and an oxidant made of one of sodium nitrate andsodium nitrite; heating at least one of the oxidant and the tungstencontaining material; and continuously producing a reaction product bybringing the tungsten containing material and the oxidant into contactwith each other in an atmosphere containing oxygen.

In the method for producing sodium tungstate according to the presentinvention, it is preferable that a heating temperature in the step ofheating is 700° C. or more and 850° C. or less.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that at least a part of thereaction product is removed from a contact interface between thetungsten containing material and the oxidant.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that a total amount of theoxidant to be supplied is 1.2 times or more and 4 times or less of atotal mass of the tungsten containing material.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that the tungsten containingmaterial is a tungsten containing scrap from which a part of a surfaceis removed.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that, in the step ofcontinuously producing the reaction product, the tungsten containingmaterial is contained in a holding container, and at least a part of thetungsten containing material contained in the holding container isimmersed in a melt containing at least one of the oxidant and thereaction product.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that at least half of thetungsten containing material is immersed in the melt.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that an amount of the tungstencontaining material immersed in the melt is kept approximately constant.

Furthermore, in the method for producing sodium tungstate according tothe present invention, it is preferable that at least a part of nitrogenoxide contained in an exhaust gas generated together with the reactionproduct is decomposed to set a concentration of the nitrogen oxide inthe exhaust gas to be continuously at 150 ppm or less.

Furthermore, it is preferable that the method for producing sodiumtungstate according to the present invention includes the steps ofhydrolyzing an oxoacid sodium salt produced by dissolving the reactionproduct in water; and filtering out a water-insoluble matter produced byhydrolysis to extract sodium tungstate.

Furthermore, the present invention provides a method for collectingtungsten including one of the above-described methods for producingsodium tungstate.

Furthermore, the present invention provides an apparatus for producingsodium tungstate. The apparatus includes a holding container for holdinga tungsten containing material; a supply port for supplying sodiumnitrate or sodium nitrite; an inlet port through which outside air flowsinto; an outlet port through which gas generated by a reaction isdischarged; a reaction container capable of holding a melt containing areaction product produced by supplying sodium nitrate or sodium nitriteonto a surface of the tungsten containing material; and a discharge portthrough which the melt can be discharged to outside of the reactioncontainer. The holding container is rotatably disposed within thereaction container. The supply port is disposed above the reactioncontainer. The discharge port is disposed below the reaction container.

Furthermore, the present invention provides a method for producing asodium tungstate aqueous solution. The method includes the steps ofintroducing a reductant into a melt containing a reaction productobtained by heating at least one of a tungsten containing material andan oxidant made of one of sodium nitrate and sodium nitrite to cause areaction between the oxidant and the tungsten containing material; anddissolving, in water, the melt into which the reductant was introduced.

In the method for producing a sodium tungstate aqueous solutionaccording to the present invention, it is preferable that a temperatureof the melt containing the reaction product into which the reductant isintroduced is 700° C. or more and 850° C. or less.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that thereductant includes a soft scrap.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that aroasted soft scrap is introduced together with the reductant.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that theroasted soft scrap contains tungsten oxide.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that anamount of the reductant to be introduced is 0.05 times or more and 0.45times or less of a mass of the melt containing the reaction product.

Furthermore, it is preferable that the method for producing a sodiumtungstate aqueous solution according to the present invention includesthe step of filtering out an insoluble matter produced by dissolving themelt in water.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that anacid is added to an aqueous solution obtained by dissolving the melt inwater, to adjust a pH to fall within a range of 8.5 or more and 10 orless.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that apressure is applied to inside of a container containing the melt tocause the melt to be pushed out and introduced into a containercontaining water.

Furthermore, in the method for producing a sodium tungstate aqueoussolution according to the present invention, it is preferable that acontainer containing the melt is tilted to cause the melt to beintroduced into a container containing water.

Furthermore, the present invention provides a method for collectingtungsten which includes one of the above-described methods for producinga sodium tungstate aqueous solution.

Effects of the Invention

According to the present invention, a method for producing sodiumtungstate that allows efficient production of sodium tungstate, a methodfor collecting tungsten using the method, and an apparatus for producingsodium tungstate can be provided.

Furthermore, according to the present invention, a method for producinga sodium tungstate aqueous solution that allows production of a sodiumtungstate aqueous solution of high purity and a method for collectingtungsten using the method can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of an apparatus forproducing sodium tungstate in accordance with the first embodiment.

FIG. 2 is a flowchart showing an example of a method for collectingtungsten in accordance with the first embodiment.

FIG. 3 is a photograph of a porcelain crucible after supplying sodiumnitrate by each constant amount for 1.2 hours to produce sodiumtungstate which is then cooled.

FIG. 4 is a photograph of the porcelain crucible after supplying sodiumnitrate by each constant amount for 0.6 hours to produce sodiumtungstate which is then cooled.

FIG. 5 is a photograph of the porcelain crucible after supplying sodiumnitrate by each constant amount for 0.3 hours to produce sodiumtungstate which is then cooled.

FIG. 6 is a schematic configuration diagram of an apparatus forproducing sodium tungstate in accordance with the second embodiment.

FIG. 7 is a schematic configuration diagram of an apparatus forproducing sodium tungstate in accordance with the third embodiment.

FIG. 8 is a schematic configuration diagram after tilting a holdingfurnace of the apparatus for producing sodium tungstate in accordancewith the third embodiment.

FIG. 9 is a diagram showing the relationship between the concentrationof Cr and V in the aqueous solution and the ratio of the amount of theintroduced soft scraps to the mass of the melt of the reaction productin Example 36.

FIG. 10 is an X-ray diffraction pattern of a solid matter remainedadhered to a reaction container in Example 37.

FIG. 11 is an X-ray diffraction pattern of residues obtained byfiltering the aqueous solution from the melt obtained by introducing amixture of WC powder and WO₃ powder in Example 37.

MODES FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will be hereinafter described.In the accompanying drawings of the present invention, the same orcorresponding components are designated by the same referencecharacters.

[First Embodiment]

FIG. 1 is a schematic configuration diagram of an apparatus forproducing sodium tungstate according to the first embodiment which is anexample of the apparatus for producing sodium tungstate used in thepresent invention. The production apparatus shown in FIG. 1 includes atungsten containing material supply port 1 through which a tungstencontaining material is supplied; a tungsten containing materialintroduction pipe 1 a connected to tungsten containing material supplyport 1; a sodium nitrate supply port 2 through which sodium nitrate issupplied; a sodium nitrate introduction pipe 2 a connected to sodiumnitrate supply port 2; an outer wall portion 10 to which tungstencontaining material introduction pipe 1 a and sodium nitrateintroduction pipe 2 a are connected; a protection container 5 disposedinside outer wall portion 10; a heating device 6 disposed withinprotection container 5; a reaction container 17 disposed within heatingdevice 6; a discharge port 11 extending through protection container 5and connected to reaction container 17; a rotation shaft 7 extendingthrough outer wall portion 10 into reaction container 17; and a holdingcontainer 8 connected to rotation shaft 7 so as to be located withinreaction container 17. Furthermore, the production apparatus shown inFIG. 1 also includes a mechanism by which the gas containing oxygen canbe introduced from outside of the production apparatus shown in FIG. 1through an inlet port 15 into reaction container 17, and nitrogen oxideproduced by the above-described reaction can be discharged through anoutlet port 16 to the outside of the production apparatus shown in FIG.1.

For example, the production apparatus having the configuration shown inFIG. 1 is used to allow production of sodium tungstate as describedbelow.

First, the tungsten containing material is supplied to tungstencontaining material supply port 1 to introduce a tungsten containingmaterial 3 through tungsten containing material introduction pipe 1 ainto holding container 8 located within reaction container 17. Holdingcontainer 8 has a side formed, for example, in a mesh pattern to have aplurality of openings and also has a bottom provided with acommunication hole. The openings of the side and the communication holeof the bottom provided in holding container 8 each are sized so as tostill prevent passage of tungsten containing material 3 that is reducedin size to some extent due to dissolution.

In addition, at the time when tungsten containing material 3 is held inholding container 8, reaction container 17 may be empty or may containthe melt of the reaction product obtained by causing a reaction betweenthe tungsten containing material and sodium nitrate in advance.

Furthermore, tungsten containing material 3 only needs to be a materialcontaining tungsten, and may be, for example, WC—Co-based cementedcarbide (WC-based cemented carbide) containing tungsten carbide andcobalt, WC—TiC—TaC—Co-based cemented carbide (WC—TiC-based cementedcarbide) containing tungsten carbide, titanium carbide, tantalum carbideand cobalt, TiCN—WC—Mo₂C—Co—Ni-based cermet (TiCN-based cermet)containing titanium carbonitride, tungsten carbide, molybdenum carbide,cobalt and nickel, W—Ni-based heavy metal (W—Ni-based alloy) containingtungsten and nickel, or the like.

Furthermore, it is preferable that tungsten containing material 3 is atungsten containing scrap from which a part of the surface is removed.The surface of the tungsten containing product such as a cementedcarbide tool may be covered by a thin film such as TiC, TiN, TiCN orAl₂O₃ that is hard and has excellent abrasion resistance. Accordingly,the waste product of the tungsten containing product (tungstencontaining scrap) from which a thin film is removed in advance is usedas a tungsten containing material, which allows efficient development ofthe reaction between the tungsten containing material and the moltensalt of sodium nitrate which will be described later.

It is preferable that the tungsten containing scrap is a hard scrap ofthe tungsten containing product such as a cemented carbide tool. In thiscase, tungsten containing material 3 can be held in holding container 8with stability, which tends to allow stable development of the reactionbetween tungsten containing material 3 and the molten salt of sodiumnitrate, as described later.

In this case, the hard scrap of the tungsten containing product means asolid waste product such as a tungsten containing product itself whichcannot be used any more due to chipping, abrasion and the like, a partof the tungsten containing product separated from the main body due tochipping, and the like.

Then, reaction container 17 is heated by heating device 6 to heattungsten containing material 3 contained in holding container 8, inwhich case tungsten containing material 3 is heated to a temperatureequal to or higher than the temperature at which sodium nitrate ismelted.

Then, holding container 8 containing tungsten containing material 3 isrotated within reaction container 17 to supply sodium nitrate 4 intosodium nitrate supply port 2. This allows sodium nitrate 4 to besupplied through sodium nitrate introduction pipe 2 a onto the surfaceof tungsten containing material 3 heated as described above.Consequently, sodium nitrate 4 brought into contact with the surface oftungsten containing material 3 is melted into molten salt, which is thenreacted with tungsten containing material 3, to thereby produce areaction product containing sodium tungstate.

In other words, since reaction container 17 is heated to the temperatureequal to or higher than the temperature at which sodium nitrate can bemaintained in the state of molten salt, the exposed surface of tungstencontaining material 3 is also heated to the temperature approximatelysimilar thereto. Accordingly, for example, even when sodium nitrate in asolid state such as a powdery state is supplied onto the surface oftungsten containing material 3, sodium nitrate is melted on the surfaceof tungsten containing material 3 into molten salt. Then, a reactionoccurs between the molten salt of sodium nitrate and the surface oftungsten containing material 3 brought into contact therewith, therebyproducing a reaction product containing sodium tungstate. It is to benoted that sodium nitrate may be supplied in the state of molten salt.

For example, when tungsten containing material 3 is WC—Co-based cementedcarbide (WC-based cemented carbide), the reaction represented by thefollowing expression (1) corresponds to the most effectively developedmain reaction among the reactions between tungsten containing material 3and the molten salt of sodium nitrate.2WC(s)+6NaNO₃(l)+½O₂(g)→2Na₂WO₄(l)+6NO(g)+2CO₂(g)+Na₂O(l)  (1)

In addition, when the reaction represented by the above-describedexpression (1) is carried out by oxygen which is supplied fromoxygen-containing atmosphere and sodium nitrate, oxygen is insufficientfor the reaction in the case of oxygen obtained from sodium nitrate canbe additionally supplied from the oxygen-containing atmosphere.Consequently, while suppressing consumption of expensive sodium nitrate,the reaction can be smoothly developed to allow efficient production ofsodium tungstate which is a main product of the above-describedreaction.

It is to be noted that the oxygen-containing atmosphere is notparticularly limited as long as the atmosphere contains oxygen, but maybe, for example, an air atmosphere and the like.

For example, in the case where tungsten containing material 3 containsCo, Ni, TiC, TiN, or TaC, an oxoacid sodium salt such as NaCO₂O₄,NaNiO₂, Na₈Ti₅O₁₄ or NaTaO₃ may be produced, in addition to sodiumoxide, as a by-product of the above-described reaction.

The reaction product produced by the reaction between tungstencontaining material 3 and the molten salt of sodium nitrate 4 asdescribed above becomes a melt which is then introduced as melt 9together with the molten salt of unreacted sodium nitrate from thesurface of tungsten containing material 3 through the openings of theside and the communication hole of the bottom in holding container 8into reaction container 17.

Furthermore, melted sodium nitrate 4 which is contained in an unreactedstate in melt 9 reacts with tungsten containing material 3 contained inholding container 8 through the openings of the side and thecommunication hole of the bottom in holding container 8. This reactionleads to production of a melt of the reaction product containing sodiumtungstate, which is then further introduced into melt 9.

Furthermore, as the tungsten containing material and the molten salt ofsodium nitrate are consumed in accordance with development of theabove-described reaction, at least one of tungsten containing material 3and sodium nitrate 4 is continuously or intermittently supplied.

In this case, tungsten containing material 3 and sodium nitrate 4 aresupplied by continuously or intermittently at least one of tungstencontaining material 3 and sodium nitrate 4 so as to keep the reactionspeed enough to allow control of the reaction between the tungstencontaining material and the molten salt of sodium nitrate.

Then, since the amount of melt 9 containing the reaction productcontaining sodium tungstate in reaction container 17 increases inaccordance with development of the above-described reaction, melt 9 iscontinuously or intermittently discharged through discharge port 11 tothe outside of reaction container 17.

In this way, at least one of tungsten containing material 3 and sodiumnitrate 4 is continuously or intermittently supplied so as to keep thereaction speed enough to allow control of the reaction between tungstencontaining material 3 and the molten salt of sodium nitrate 4. Then,melt 9 containing the reaction product containing sodium tungstateproduced by the reaction between tungsten containing material 3 and themolten salt of sodium nitrate 4 is continuously or intermittentlydischarged through discharge port 11. Consequently, the reaction productcontaining sodium tungstate can be efficiently and continuously producedwith safety.

It is to be noted that tungsten containing material 3 can be supplied,for example, by supplying the tungsten containing material into tungstencontaining material supply port 1 to introduce tungsten containingmaterial 3 through tungsten containing material introduction pipe 1 ainto holding container 8.

Furthermore, sodium nitrate 4 can be supplied, for example, by supplyingsodium nitrate 4 into sodium nitrate supply port 2 to introduce sodiumnitrate 4 through sodium nitrate introduction pipe 2 a onto the surfaceof tungsten containing material 3 contained in holding container 8.

Furthermore, the timing for supplying tungsten containing material 3 canbe determined, for example, by calculating the remaining amount oftungsten containing material 3 through measurement of the mass,detection of the liquid level position using laser or the like.

Furthermore, the timing for supplying sodium nitrate 4 can bedetermined, for example, by calculating the remaining amount of sodiumnitrate 4 through measurement of the mass, detection of the liquid levelposition using laser or the like.

Furthermore, when the amount of melt 9 contained in reaction container17 increases in accordance with development of the above-describedreaction, tungsten containing material 3 contained in holding container8 may be immersed in melt 9.

In this case, when entire tungsten containing material 3 is immersed inmelt 9, the amount of produced nitrogen oxide is increased to cause alarge quantity of gas to be generated within melt 9. This may causeoverflow of melt 9 from reaction container 17. Accordingly, it ispreferable that only a part of tungsten containing material 3 isimmersed in melt 9.

It is preferable that at least half of tungsten containing material 3 isimmersed in melt 9. In this case, the exposed portion of tungstencontaining material 3 which is not covered by melt 9 is brought intodirect contact with sodium nitrate, which leads to an increase in thereaction speed. However, the portion exposed from melt 9 facilitatessmooth separation of the generated gas, and accordingly, overflow isless likely to occur. Consequently, the reaction with the molten salt ofsodium nitrate can be developed with safety. Furthermore, the reactionspeed is relatively low in the portion of tungsten containing material 3immersed in melt 9, which allows moderate development of the reactionwith the molten salt of sodium nitrate in melt 9. Therefore, the amountof gas to be generated is reduced to suppress overflow, which allows thereaction to proceed with safety. Accordingly, the reaction productcontaining sodium tungstate can be more efficiently produced.

It is to be noted that the phrase “at least half of the tungstencontaining material is immersed in the melt” used herein means that theportion corresponding to at least half of the maximum height of a groupof tungsten containing materials is immersed in the melt in the heightdirection of the group of tungsten containing materials.

Furthermore, it is preferable that the amount of tungsten containingmaterial 3 immersed in melt 9 is kept approximately constant. In thiscase, since the reaction between the tungsten containing material andthe molten salt of sodium nitrate can be performed at an approximatelyconstant reaction speed, the reaction product containing sodiumtungstate can be produced with enhanced safety.

The amount of tungsten containing material 3 immersed in melt 9 can beadjusted, for example, by moving rotation shaft 7 connected to holdingcontainer 8 in the upward and downward directions and/or by adjustingthe amount of melt 9 in reaction container 17 by discharging melt 9through discharge port 11.

Furthermore, the melting point of sodium tungstate is 698° C.Accordingly, when the temperature of sodium tungstate produced by theabove-described reaction is decreased below 698° C., sodium tungstatewill be present as a solid phase at the contact interface between thetungsten containing material and the molten salt of sodium nitrate. Inthis case, for example as described above, when sodium nitrate 4 in asolid state is supplied onto the exposed surface of tungsten containingmaterial 3, sodium nitrate having a melting point of 308° C. is meltedon the surface of sodium tungstate existing as a solid phase, whichmakes it difficult to reach the surface of tungsten containing material3. Consequently, the reaction speed of the above-described reaction maybe decreased.

Thus, in order to promote the reaction between the tungsten containingmaterial and the molten salt of sodium nitrate, it is preferable to heatat least one of the tungsten containing material and the molten salt ofsodium nitrate to a temperature of 700° C. or higher. In this case, atleast a part of sodium tungstate can be brought into a state of a melt.In addition, while removing the melt of sodium tungstate from thecontact interface between the tungsten containing material and themolten salt of sodium nitrate, the reaction between the tungstencontaining material and the molten salt of sodium nitrate can bedeveloped. Accordingly, it is more likely that sodium tungstate can bemore efficiently produced.

In addition, in the case where the reaction product (sodium tungstateand its by-product) is brought into a state of a melt, the melt of thereaction product can also be removed from the surface of the tungstencontaining material and introduced into melt 9.

Furthermore, when the tungsten containing material is TiCN-based cermetgenerally including a tungsten content of 20% or less, the amount ofproduced sodium tungstate is decreased. In this case, the melt isproduced at a high temperature (850° C.) from the reaction productproduced by the reaction between the tungsten containing material andthe molten salt of sodium nitrate. In contrast, when the heatingtemperature during the reaction between the tungsten containing materialand the molten salt of sodium nitrate is excessively high, the reactionis developed to the level at which the reaction cannot be controlled.Therefore, it is preferable that the heating temperature for at leastone of the tungsten containing material and the molten salt of sodiumnitrate is set at 850° C. or lower during the reaction between thetungsten containing material and the molten salt of sodium nitrate.

Furthermore, it is preferable that the total amount of sodium nitrate tobe supplied is 1.2 times or more and 4 times or less of the total massof the tungsten containing material. For example, according to theabove-described expression (1), the ratio (B/A) between the mass (A) ofthe tungsten containing material and the mass (B) of sodium nitratewhich contributes to the above-described reaction is approximately 1.3.

However, the tungsten containing material made of WC-based cementedcarbide also contains a bonding metal. Accordingly, when theabove-described ratio (B/A) is actually 1.2 or more, it is more likelythat the tungsten containing material can be completely reacted.

Furthermore, since it is considered that the reaction of TiC or TiCNproceeds as in the following expression (2), it is also considered that,when using the tungsten containing material made of TiCN-based cermetthat contains TiCN as a main component and added with a less amount ofWC, a large amount of sodium nitrate is required as compared with thecase where the tungsten containing material made of the above-describedWC-based cemented carbide is used.5TiC(s)+8NaNO₃(l)+4O₂(g)→Na₈Ti₅O₁₄(s)+8NO(g)+5CO₂(g)  (2)

The complete reaction of the tungsten containing material requires theabove-described ratio (B/A) to be at least 4. Furthermore, even when theabove-described ratio (B/A) exceeds 4, the tungsten containing materialcan be completely reacted, in which case the reaction may excessivelyproceed or there may remain sodium nitrate that is not used for thereaction. This may cause needless consumption of expensive sodiumnitrate. Therefore, it is preferable that the above-described ratio(B/A) is 4 or less.

Furthermore, it was examined what change would occur when the timeperiod for supplying sodium nitrate is changed while the total amount ofsodium nitrate addition is fixed. More specifically, 70 g of WC—Co-basedcemented carbide (WC-based cemented carbide) was heated to 720° C.within the porcelain crucible. Then, 120 g of sodium nitrate was addedgradually by each constant amount onto the surface of WC—Co-basedcemented carbide (WC-based cemented carbide) in respective cases for 1.2hours, 0.6 hours and 0.3 hours, to produce sodium tungstate. FIGS. 3 to5 each show the porcelain crucible at the time when sodium tungstate wasproduced and then cooled to a room temperature. It is to be noted thatFIGS. 3, 4 and 5 each show the state of the above-described cooledporcelain crucible at the time when 120 g of sodium nitrate was added byeach constant amount for 1.2 hours (FIG. 3), 0.6 hours (FIG. 4) and 0.3hours (FIG. 5), respectively.

It is to be noted that the reaction rate of WC-Co-based cemented carbide(WC-based cemented carbide) (100×{(mass of WC-based cemented carbidebefore reaction) −(remaining amount of WC-based cemented carbide afterreaction)}/(mass of WC-based cemented carbide before reaction)) was 99%(1.2 hours), 92% (0.6 hours) and 86% (0.3 hours). The reaction betweensodium nitrate and WC—Co-based cemented carbide (WC-based cementedcarbide) became active in accordance with addition of sodium nitrate fora short time period, and the exhaust gas by the reaction caused overflowof the reaction product. Consequently, the reaction rate was lowered.

When further examination has been made based on the above-describedresults, it has been found that sodium nitrate is supplied for 1 hour ormore, which allows a high reaction rate to be achieved without causingoverflow of the reaction product. Furthermore, it has been found thatsodium nitrate is supplied over a time period longer than 4.5 hours,which causes an increase in energy consumption which is uneconomical.

Furthermore, it is preferable that sodium nitrate 4 is supplied so as tobe uniformly distributed throughout tungsten containing material 3. Forexample, when a component such as a sieve is provided in the upperportion of holding container 8, or when the relative positionalrelationship between sodium nitrate introduction pipe 2 a serving as asupply port for substantially supplying sodium nitrate 4 into reactioncontainer 17 and holding container 8 containing tungsten containingmaterial 3 is set variable, sodium nitrate 4 can be supplied so as to beuniformly distributed throughout tungsten containing material 3.

In addition, the mechanism for changing the relative positionalrelationship between sodium nitrate introduction pipe 2 a and holdingcontainer 8 may includes, for example, the mechanism for moving sodiumnitrate introduction pipe 2 a, the mechanism for rotating holdingcontainer 8 about rotation shaft 7, the mechanism for moving holdingcontainer 8 in the upward and downward directions while rotating holdingcontainer 8 about rotation shaft 7, or the like.

Furthermore, as shown in the reaction represented by each of theabove-described expressions (1) and (2), when a reaction occurs betweenthe tungsten containing material and the molten salt of sodium nitrate,the exhaust gas containing toxic nitrogen oxide is generated as aby-product. For example, as in the reaction represented by the followingexpression (3), nitrogen monoxide (NO) generated by the reaction of eachof the above-described expressions (1) and (2) can be decomposed intonitrogen and water by mixing ammonia gas in the presence of an oxidecatalyst of vanadium or titanium of a high temperature.4NO+4NH₃+O₂→4N₂+6H₂O  (3)

It is preferable that the concentration of the nitrogen oxide in theexhaust gas generated together with a reaction product is set to becontinuously at 150 ppm or lower. In this case, the exhaust gascontaining a low concentration of toxic nitrogen oxide can bedischarged. Accordingly, the safety of the operation can be improvedwhile an adverse effect is less likely to be exerted on the environment.

In addition, the reaction product other than the exhaust gas generatedby the reaction between the tungsten containing material and the moltensalt of sodium nitrate can be introduced into melt 9 as a melt. Then,melt 9 containing the reaction product is discharged through dischargeport 11 and introduced into a container 18 disposed outside of reactioncontainer 17.

In this way, melt 9 within reaction container 17 is discharged throughdischarge port 11, which allows adjustment of the amount of melt 9contained in reaction container 17, and also allows continuousdevelopment of the reaction between the tungsten containing material andthe molten salt of sodium nitrate without stopping the operation.Therefore, sodium tungstate can be more efficiently produced as comparedwith the method disclosed in Patent Document 1 that requires thereaction to be stopped for every batch processing.

Furthermore, when the amount of melt 9 within reaction container 17 isincreased or decreased without keeping it constant, it is preferablethat holding container 8 containing tungsten containing material 3 ismovable in the vertical direction. For example, in the case where melt 9in reaction container 17 is collectively transferred to externalcontainer 18 at the time when the amount of melt 9 is increased to someextent, and also in the case where holding container 8 is configured tohave a bottom provided with a communication hole and a side formed in amesh pattern as described above, it is preferable that holding container8 is also configured such that it can be raised in accordance with anincrease in the amount of melt 9. Furthermore, holding container 8 isconfigured to have a fixed depth and also configured to have a side inwhich only an upper portion is formed in a mesh pattern. This causesmelt 9 accumulated to the level equal to or greater than the prescribedamount to be overflown through the mesh, to thereby keep the liquidlevel constant. Thus, the molten salt of the unreacted sodium nitratethat did not react in the upper portion of the side of holding container8 may be reacted with tungsten containing material 3 contained in thelower portion of holding container 8.

Furthermore, in order to keep the amount of melt 9 constant, forexample, discharge port 11 connected to outside of reaction container 17may be configured in a siphon type to thereby allow the height of theliquid level of melt 9 to be kept constant.

Furthermore, a protection container 13 provided with a heating device 14is provided outside of container 18 disposed external to reactioncontainer 17. A stirrer which is not shown is disposed inside ofcontainer 18. Furthermore, container 18 is movable into and out of outerwall portion 10 shown in FIG. 1 by a plurality of rollers 12 arranged onthe bottom of protection container 13.

In addition, container 18 containing melt 9 including the reactionproduct produced by the reaction between the tungsten containingmaterial and the molten salt of sodium nitrate is moved to the outsideof outer wall portion 10 of the apparatus shown in FIG. 1, and areductant is introduced into melt 9 contained in container 18.

Introduction of the reductant into melt 9 allows reduction in a valenceof the metallic elements constituting the ions of metal peroxidecorresponding to impurities in melt 9. Consequently, when melt 9 isdissolved in water in the subsequent process, the metallic elementsregarded as impurities can be precipitated in the state of an oxide as awater-insoluble matter.

In other words, melt 9 contained in container 18 may contain peroxideions of impurity metal such as chromium and vanadium, for example. Thesemetal peroxide ions are changed into water-soluble hexavalent orpentavalent oxoacid sodium salt, but not deposited as a precipitate whenmelt 9 is cooled and dissolved in water. Accordingly, these metalperoxide ions cannot readily be separated by filtration and the like.

Thus, a reductant having a reduction action is introduced in advancebefore melt 9 containing these metal peroxide ions is dissolved inwater. This causes a decrease in the valence of the metallic elementsconstituting metal peroxide ions to achieve water-insoluble metaloxides. In addition, in the case where melt 9 containing theabove-described metal oxides is dissolved in water, these metal oxidesare insoluble in water and deposited as precipitates. Therefore, thesemetal oxides can readily be separated from the sodium tungstate aqueoussolution by filtration and the like.

For example, in the case where melt 9 contains chromium which is animpurity metal as a chromate ion (CrO₄ ²⁻) which is a metal peroxideion, carbon (C), tungsten (W) or tungsten carbide (WC) is introduced asa reductant, thereby allowing production of water-insoluble dichromiumtrioxide (Cr₂O₃), for example, as represented in the followingexpressions (4) to (6).2NaCrO₄+2C→Na₂O+Cr₂O₃+2CO₂  (4)2NaCrO₄+W→Na₂WO₄+Cr₂O₃+½O₂  (5)2NaCrO₄+WC+½O₂→Na₂WO₄+Cr₂O₃+CO₂  (6)

For example, in the case where melt 9 contains vanadium which is animpurity metal as a vanadate ion (VO³⁻) which is a metal peroxide ion,carbon (C) or tungsten (W) is introduced as a reductant, therebyallowing production of water-insoluble divanadium trioxide (V₂O₃), forexample, as represented in the following expressions (7) to (9).2NaVO₃+C→V₂O₃+Na₂CO₃  (7)14NaVO₃+4W→4Na₂WO₄+6V₂O₃+(Na₂O)₃(V₂O₅)  (8)14NaVO₃+4WC→6V₂O₃+4Na₂WO₄+4C+(Na₂O)₃(V₂O₅)  (9)

As described above, a reductant is introduced into melt 9 containingperoxide ions of the impurity metal before water is introduced, tothereby produce an oxide of water-insoluble impurity metal in advance.This allows melt 9 to exist as a precipitate when melt 9 is dissolved inwater and also allows the precipitate to be readily separated byfiltration and the like. Therefore, the sodium tungstate aqueoussolution of high purity can be obtained.

It is to be noted that the reductant is not particularly limited as longas it allows a decrease in the valence of the metallic elementsconstituting peroxide ions of the impurity metal in melt 9.Specifically, it is preferable to use soft scraps including at least oneselected from the group consisting of carbon, tungsten and tungstencarbide, in which case the soft scraps may further contain tungstenoxide. This tends to allow a further decrease in the content of theimpurities other than sodium tungstate in melt 9.

For example, there is a case where soft scraps contain tungsten carbideand tungsten and further contain tungsten oxide. This is preferable inthe point that the reaction represented in each of the followingexpressions (10) to (12) allows production of sodium tungstate fromtungsten carbide and sodium oxide and also allows a decrease in thecontent of sodium peroxide or sodium oxide which exists as impuritieswith instability.WC+Na₂O+5/2O₂→Na₂WO₄+CO₂  (10)W+Na₂O₂+O₂→Na₂WO₄  (11)WO₃+Na₂O₂→Na₂WO₄+½O₂  (12)

The soft scraps used herein mean powdery substances. Particularly, thesoft scraps of a tungsten containing product correspond to collectedpowder such as scraps produced during production of the tungstencontaining product and scraps produced by the processing using thetungsten containing product.

Furthermore, it is preferable that not only the soft scraps of thetungsten containing product but also those further containing tungstenoxide are used as reductants. In this case, for example, as shown in thereaction of the above-described expressions (10) and (11), oxygenrequired for the reaction between sodium oxide and sodium peroxide canbe supplied, which tends to allow more efficient removal of sodiumoxide.

Furthermore, in order to efficiently reduce the above-described impuritymetallic elements, it is preferable that the temperature of the melt atwhich the reductant is introduced is 700° C. or higher and 850° C. orlower.

Furthermore, it is preferable that the amount of the reductant to beintroduced is 0.05 times or more and 0.45 times or less of the mass ofmelt 9 containing the reaction product, and, more preferably, 0.1 timesor more and 0.4 times or less. The amount of the reductant to beintroduced is 0.05 times or more and 0.45 times or less, particularly,0.1 times or more and 0.4 times or less, of the mass of melt 9containing the reaction product, which tends to allow sufficientreduction of the peroxidized impurity metallic elements in the reactionproduct to thereby obtain a sodium tungstate aqueous solution of highpurity.

After the above-described reductant is introduced, protection container13 and heating device 14 are removed from container 18. Melt 9 containedin container 18 is then cooled and solidified, into which water isintroduced to dissolve melt 9 in water. Consequently, the metal which isan impurity reduced by the above-described reductant is precipitated inthe state of an oxide in the aqueous solution, and water-soluble sodiumtungstate can be obtained in the state of an aqueous solution.

After the above-described reductant is introduced, protection container13 and heating device 14 are removed from container 18. Melt 9 containedin container 18 is then cooled and solidified, into which water isintroduced to dissolve melt 9 in water. Consequently, the metal which isan impurity reduced by the above-described reductant is precipitated inthe state of an oxide in the aqueous solution, and water-soluble sodiumtungstate can be obtained in the state of an aqueous solution.

Then, the aqueous solution of melt 9 is filtered to remove theabove-described metal oxide as a precipitate, thereby allowing thesodium tungstate aqueous solution of high purity to be obtained.

It is preferable to adjust a pH to fall within the range of 8.5 or moreand 10 or less by adding acid (preferably, sulfuric acid) to the aqueoussolution obtained after dissolving, in water, melt 9 used in the casewhere the impurities are removed with precipitates by adjustment of pH.In this case, Al and Si can be precipitated while suppressing elution ofCo in the aqueous solution, which allows the number of times offiltration to be reduced to simplify the process, and also allows theequipment such as a solution tank to be eliminated.

In other words, when melt 9 obtained after the above-described reductantis introduced is dissolved in water, the aqueous solution may containimpurities such as Al and Si and the residues insoluble in water maycontain Co and the like. In this case, after the residues containing Coare separated by filtration, acid is added to the aqueous solution toproduce precipitates of each of Al and Si (for example, Al(OH)₃, H₂SiO₃and the like), thereby also allowing the precipitates to be filteredagain. However, as described above, the pH is adjusted to fall withinthe range of 8.5 or more and 10 or less by adding acid to the aqueoussolution of melt 9 into which the reductant has been introduced. Thisallows Al and Si to be precipitated and contained in the residuescontaining Co without elution of Co into the aqueous solution.Accordingly, the number of times of filtration can be reduced tosimplify the process, and the equipment such as a solution tank can alsobe eliminated.

Furthermore, oxoacid sodium salt such as NaCO₂O₄, NaNiO₂, Na₈Ti₅O₁₄, orNaTaO₃ in melt 9 can be removed as described below, for example.

First, melt 9 is dissolved in water to hydrolyze the above-describedoxoacid sodium salt. This allows the above-described oxoacid sodium saltto be converted into a water-insoluble matter (for example, CO₃O₄, NiO,NiO₂, Na₃TaO₄, Ta₂O₅, and the like).

Then, the aqueous solution obtained after hydrolysis of theabove-described oxoacid sodium salt is filtered to thereby allow thewater-insoluble matter (for example, CO₃O₄, NiO, NiO₂, Na₃TaO₄, Ta₂O₅,and the like) to be separated from the sodium tungstate aqueoussolution. Consequently, the sodium tungstate aqueous solution of highpurity can be obtained.

As described above, the sodium tungstate aqueous solution of high puritycan be obtained in the present invention.

FIG. 2 is a flowchart showing a method for collecting tungsten in thefirst embodiment which is an example of the method for collectingtungsten according to the present invention. With reference to FIG. 2,an example of the method for collecting tungsten in accordance with thepresent invention using the above-described method for producing sodiumtungstate will be hereinafter described.

First, the tungsten containing material and the molten salt of sodiumnitrate are reacted with each other in an oxygen-containing atmosphere,to obtain, for example, a melt-like reaction product (Na₂WO₄, Na₂O, andNaCO₂O₄ and the like). In this case, sodium nitrate can be effectivelyused by supplying oxygen from the oxygen-containing atmosphere.

Then, the by-product (Na₂O, NaCO₂O₄ and the like) of the reactionproduct obtained as described above is reduced by a reductant asdescribed above, and Na₂O and Na₂O₂ are also removed by the reactionrepresented by each of the above-described expressions (10) to (12) andthe like, to thereby obtain a product (Na₂WO₄, NaCO₂O₄ and the like).

Then, the product obtained as described above (Na₂WO₄, NaCO₂O₄ and thelike) is dissolved in water, into which acid is added for hydrolysiswhich is followed by filtration. Consequently, water-insolubleprecipitates (for example, oxides such as Co, Ta or Ti) are separatedfrom the sodium tungstate aqueous solution. The above-describedprocesses can be performed using the apparatus shown in FIG. 1, asdescribed above.

It is to be noted that the above-described precipitates (for example,oxides such as Co, Ta or Ni) are collected after separation.

With regard to the sodium tungstate (Na₂WO₄) aqueous solution, forexample, ion exchange using an ion exchange resin is employed tosubstitute ammonium ions for sodium ions to achieve an ammoniumtungstate ((NH₄)₂WO₄) aqueous solution.

Then, the ammonium tungstate ((NH₄)₂WO₄) aqueous solution isconcentrated to cause crystallization of ammonium paratungstate (APT).At this time, the ammonia (NH₃) gas generated during concentration ofthe ammonium tungstate ((NH₄)₂WO₄) aqueous solution is collected.

Then, ammonium paratungstate (APT) is calcined to obtain tungsten oxide(WO₃). At this time, the ammonia (NH₃) gas generated during calcinationof ammonium paratungstate (APT) is also collected.

Then, tungsten oxide (WO₃) is reduced to obtain tungsten (W), whichallows tungsten (W) to be collected.

Although the description has been made in the above with regard to thecase where sodium nitrate is used as an oxidant, sodium nitrite may beused as an oxidant in place of sodium nitrate in the present invention.

Furthermore, although the description has been made in the above withregard to the case where sodium nitrate is supplied into the tungstencontaining material, the tungsten containing material may also besupplied into sodium nitrate in the present invention.

[Second Embodiment]

FIG. 6 is a schematic configuration diagram of an apparatus forproducing sodium tungstate in accordance with the second embodimentwhich is another example of the apparatus for producing sodium tungstateused in the present invention. The production apparatus shown in FIG. 6is characterized by the following points (a) to (h).

(a) With regard to the tungsten containing material, a storage unit 1 bcapable of storing, for example, the tungsten containing material ofseveral 100 kilograms is provided above tungsten containing materialsupply port 1. The tungsten containing material is automaticallysupplied through a supply conveyor (not shown) and tungsten containingmaterial supply port 1 into tungsten containing material introductionpipe 1 a such as a quantitative supply hopper. Consequently, the amountof the tungsten containing material to be dissolved can be increased toallow improvement in productivity of sodium tungstate.

(b) Sodium nitrate is supplied through sodium nitrate supply port 2 intosodium nitrate introduction pipe 2 a, for example, using a flexiblecontainer (not shown) of several 100 kilograms. This allows an increasedefficiency of the operation for supplying sodium nitrate 4.

(c) A holding furnace 41 is disposed obliquely below discharge port 11.This allows an automatic supply device 23 to be disposed above holdingfurnace 41. Thus, the processing of melt 9 by introduction of thereductant can be performed simultaneously while continuously dissolvingthe tungsten containing material. Consequently, the productivity ofsodium tungstate can be improved.

(d) Melt 9 discharged through discharge port 11 is introduced intoholding furnace 41 through a trough 21. Trough 21 is not enclosed butopened. Accordingly, the problem of clogging of the pipe resulting fromsolidification of melt 9 can be solved, and the maintenance can also bereadily performed when melt 9 is adhered to the bottom of trough 21.

(e) Automatic supply device 23 for soft scraps is provided as a devicefor supplying a reductant. Soft scraps 25 are introduced from automaticsupply device 23 into holding furnace 41. In that case, it is preferablethat the roasted powder of the soft scraps obtained as oxides byroasting the soft scraps in advance is also introduced into holdingfurnace 41 together with soft scraps 25. This is because the processingof melt 9 by introduction of the soft scraps can be more efficientlyperformed when a part of the soft scraps is roasted and then oxidized(roasted powder) in advance before it is introduced into holding furnace41. It is to be noted that the reaction caused by introduction of softscraps 25 is represented, for example, by the following expression (10)and the reaction caused by introduction of the soft scraps (roastedpowder) is represented, for example, by the following expression (12′).Furthermore, automatic supply device 23 is provided with a function forquantitative supply and is also capable of controlling the introductiondistribution.WC+Na₂O+5/2O₂→Na₂WO₄+CO₂  (10)WO₃+Na₂O→Na₂WO₄  (12′)

(f) Holding furnace 41 is provided with a stirrer 27 and also providedat its upper portion with a dust-collecting outlet port 22. When melt 9is stirred by stirrer 27, the reaction of melt 9, soft scraps 25 andsoft scraps (roasted powder: tungsten oxide) can be promoted.Furthermore, dust-collecting outlet port 22 allows soft scraps 25 andthe soft scraps (roasted powder) floating above melt 9 to be dischargedto the outside of holding furnace 41.

(g) It is assumed that melt 9 is discharged in a pressure extrusionmanner from holding furnace 41 after the processing by introduction ofsoft scraps 25 and the soft scraps (roasted powder). The pressureextrusion manner is employed, which eliminates the need to use a valveto control the amount of melt 9 to be discharged from holding furnace41. Accordingly, clogging in the valve can be effectively prevented.When melt 9 is discharged from holding furnace 41 in a pressureextrusion manner, gas is introduced through a gas introduction port 28into holding furnace 41 to raise the pressure within holding furnace 41.Then, the above-described processed melt 9 is discharged through a meltdischarge passage 29 to the outside of holding furnace 41. Consequently,the amount of melt 9 to be discharged can be freely controlled by theamount of the gas to be introduced through gas introduction port 28.

(h) A water dissolution bath 42 including container 18 containing water39 in advance and stirrer 37 is disposed obliquely below holding furnace41. This allows melt 9 to directly flow from holding furnace 41 intowater 39, thereby facilitating dissolution of melt 9 into water 39through stir by stirrer 37. Consequently, the productivity of sodiumtungstate can be enhanced.

Thus, the amount of melt 9 to be discharged is freely controlled, tothereby allow melt 9 in the appropriately dispersed state to bedissolved in water 39 so as to prevent melt 9 from being solidified intoa large mass. Consequently, it becomes possible to prevent a large masswhich is a solidified melt 9 from reaching the bottom of container 18,thereby also preventing the contents in container 18 from coming to aboil locally starting at the bottom thereof. Furthermore, melt 9 canalso be efficiently dissolved in water 39 by appropriately distributingmelt 9.

Since the description in the present embodiment other than that in theabove is the same as that of the first embodiment, description thereofwill not be repeated.

[Third Embodiment]

FIG. 7 is a schematic configuration diagram of an apparatus forproducing sodium tungstate in accordance with the third embodiment whichis a still another example of the apparatus for producing sodiumtungstate used in the present invention. The production apparatus shownin FIG. 7 is characterized by the following point (i) instead of theabove-described point (g) in the second embodiment.

(i) Melt 9 is discharged from holding furnace 41 to water dissolutionbath 42 by means of a tilt mechanism. In other words, holding furnace 41(a soft scrap supply mechanism and a stirrer are not shown) into whichsoft scraps 25 and soft scraps (roasted powder) have been introduced is,for example, separated from the soft scrap supply mechanism and thestirrer, and then tilted as shown in the schematic configuration diagramin FIG. 8. This causes melt 9 contained in holding furnace 41 to bedischarged into water dissolution bath 42. Accordingly, the equipmentcan be simplified as compared with the above-described pressureextrusion type in (g), and the cost of the apparatus can also bereduced.

EXAMPLES Examples 1 to 22

First, a reaction container made of stainless steel having an innervolume of 1.7 liters was disposed within an electric furnace which canbe uniformly heated. Various types of reactants (A) shown in thefollowing Table 1 were introduced into the reaction container. Then, thereactants (A) were heated in the atmosphere to each reaction temperatureshown in Table 1.

Then, various types of reactants (B) shown in the following Table 1 weregradually added by a small amount at certain time intervals for theirrespective supply time periods shown in the following Table 1 to thereactants (A) heated within the reaction container, to thereby cause areaction between the reactants (A) and the reactants (B).

In Examples 1 to 22, at least one of the conditions including reactant(A), reactant (B), mass ratio of the reactants (B/A), supply time period(time), and reaction temperature (° C.) was changed for undergoing theabove-described reactions. Then, evaluations were made with regard tothe reaction rate of the tungsten containing material and whetheroverflow of the reaction product from the reaction container occurred ornot. The results are shown in Table 1.

It is to be noted that the mass ratio (B/A) shown in Table 1 representsthe ratio of the total mass of the amount of the reactant (B) suppliedto the reaction container with respect to the total mass of the reactant(A) contained in the reaction container.

Furthermore, the reaction rate (%) shown in Table 1 was calculated bythe following expression (13).Reaction rate (%)=(100×{(mass of tungsten containing material beforereaction)−(remaining amount of tungsten containing material afterreaction)}/(mass of tungsten containing material before reaction))  (13)

Furthermore, it was evaluated based on the following criteria whetheroverflow of the reaction product from the reaction container occurred ornot, as shown in Table 1.

Overflow occurred . . . It can be visually clearly confirmed that thereaction product is adhered to the outer surface of the reactioncontainer.

Overflow did not occur . . . It can be visually confirmed that littlereaction product is adhered to the outer surface of the reactioncontainer.

TABLE 1 Reaction Whether or not Reactant (A) Heated in Reactant (B)Supplied Mass Ratio Supply Time Temperature Reaction Overflow ofReaction Reaction Container to Reaction Container (B/A) Period (Time) (°C.) Rate (%) Product Occurred Example 1 WC-based Cemented Carbide NaNO₃1.2 1.2 720 98 Not Occurred Example 2 WC-based Cemented Carbide NaNO₃1.2 1.0 720 96 Not Occurred Example 3 WC-based Cemented Carbide NaNO₃1.4 4.5 750 100 Not Occurred Example 4 WC-based Cemented Carbide NaNO₃1.5 1.1 700 100 Not Occurred Example 5 WC-based Cemented Carbide NaNO₃1.2 1.0 700 95 Not Occurred Example 6 WC-based Cemented Carbide NaNO₂2.1 2.5 710 95 Not Occurred Example 7 WC—TiC-based Cemented NaNO₃ 1.51.5 770 97 Not Occurred Carbide Example 8 WC—TiC-based Cemented NaNO₃1.7 1.0 720 99 Not Occurred Carbide Example 9 TiCN-based Cermet NaNO₂4.0 1.0 850 96 Not Occurred Example 10 W—Ni-based Alloy NaNO₃ 1.2 3.0730 100 Not Occurred Example 11 NaNO₃ WC-based Cemented 0.75 2.0 730 96Not Occurred Carbide Example 12 NaNO₂ WC-based Cemented 0.45 1.5 710 94Not Occurred Carbide Example 13 NaNO₃ TiCN-based Cermet 0.25 1.0 850 93Not Occurred Example 14 WC-based Cemented Carbide NaNO₃ 1.0 1.2 720 81Not Occurred Example 15 WC-based Cemented Carbide NaNO₃ 1.0 1.0 680 72Not Occurred Example 16 WC-based Cemented Carbide NaNO₃ 1.3 0.50 750 85Occurred Example 17 WC-based Cemented Carbide NaNO₃ 1.2 0.83 680 89Occurred Example 18 WC-based Cemented Carbide NaNO₃ 1.1 0.67 720 84Occurred Example 19 WC-based Cemented Carbide NaNO₃ 1.2 0.33 720 86Occurred Example 20 WC—TiC-based Cemented NaNO₃ 1.5 0.42 730 87 OccurredCarbide Example 21 NaNO₂ WC-based Cemented 0.80 1.5 680 75 Not OccurredCarbide Example 22 NaNO₃ WC-based Cemented 0.70 0.83 720 88 OccurredCarbide

As shown in Examples 1 to 6 and 14 to 19 in Table 1, in the case wherethe above-described reactant (A) was WC-based cemented carbide, it wasconfirmed that the reaction rate of WC-based cemented carbide was highwhen the mass ratio (B/A) was 1.2 or more, and also confirmed thatoverflow of the reaction product did not occur when the supply timeperiod of the reactant (B) was 1 hour or more.

Furthermore, as shown in Examples 7, 8 and 20 in Table 1, in the casewhere the above-described reactant (A) was WC—TiC-based cementedcarbide, it was confirmed that the reaction rate of WC—TiC-basedcemented carbide was high when the mass ratio (B/A) was 1.5 or more, andalso confirmed that overflow of the reaction product did not occur whenthe supply time period of the reactant (B) was 1 hour or more.

Furthermore, as shown in Example 9 in Table 1, in the case where theabove-described reactant (A) was TiCN-based cermet, it was confirmedthat TiCN-based cermet could be sufficiently reacted when the mass ratio(B/A) was 4.

Furthermore, as shown in Example 10 in Table 1, in the case where theabove-described reactant (A) was W—Ni-based alloy, it was confirmed thatW—Ni-based alloy could be sufficiently reacted when the mass ratio (B/A)was 1.2 or more.

Furthermore, as shown in Examples 11, 12, 21 and 22 in Table 1, in thecase where the tungsten containing material (WC-based cemented carbide)was supplied to sodium nitrate (NaNO₃) or sodium nitrite (NaNO₂), it wasconfirmed that the tungsten containing material could be sufficientlyreacted when the total amount of the supplied tungsten containingmaterial was 1.2 times or more of the total mass of NaNO₃ or NaNO₂ (whenthe reciprocal number of the mass ratio (B/A) in Table 1 was 1.2 ormore).

Furthermore, as shown in Example 13 in Table 1, in the case where thetungsten containing material (TiCN-based cermet) was supplied to NaNO₃,it was confirmed that the tungsten containing material could besufficiently reacted when the total amount of the supplied tungstencontaining material was 4 times of the total mass of NaNO₃ (when thereciprocal number of the mass ratio (B/A) in Table 1 was 4), and alsoconfirmed that overflow of the reaction product did not occur when thesupply time period of the tungsten containing material was 1 hour ormore.

Furthermore, in Examples 1 to 10 and 14 to 20, when the tungstencontaining material was heated to a temperature of 700° C. or more, itwas confirmed that the reaction product was converted into a melt whichwas readily flown and fallen down from the surface of the tungstencontaining material, to thereby facilitate the development of thereaction to increase the reaction rate.

In view of the above-described results, it was confirmed that each typeof tungsten containing materials, NaNO₃ or NaNO₂ was gradually suppliedfor 1 hour or more, to cause a reaction between each type of tungstencontaining materials and the molten salt of NaNO₃ or NaNO₂ at a reactiontemperature of 700° C. or more to thereby produce the melt of thereaction product containing Na₂WO₄, with the result that the tungstencontaining material could be efficiently reacted while controlling thereaction to suppress overflow from the reaction container.

Furthermore, it was also confirmed that the reaction product containingsodium tungstate could be continuously and efficiently produced byrepeatedly performing the operations in Examples 1 to 13 shown in Table1.

Example 23

A coated chip covered with a thin film such as TiC, TiN, TiCN, or Al₂O₃was introduced into a drum which was then rotated. Thus, a part of thethin film was removed by the effects caused by the coated chips rubbedagainst each other. Then, when the coated chip from which a part of thethin film was removed was reacted as the above-described reactant (A) onthe conditions of Example 7 in Table 1, it was confirmed that a highreaction rate of 99% was achieved.

It is to be noted that removal of the thin film of the coated chip isnot limited to the method implemented by the coated chips rubbed againsteach other, but may be performed by some other chemical means and/orphysical means.

Example 24

On the conditions in Example 1, the reaction rate was examined in eachof the cases where (i) WC-based cemented carbide was not immersed in themelt of the reaction product, (ii) 50% of WC-based cemented carbide wasimmersed in the melt of the reaction product, (iii) 90% of WC-basedcemented carbide was immersed in the melt of the reaction product, and(iv) WC-based cemented carbide was completely immersed in the melt ofthe reaction product (depth of about 50 mm). The reaction rate wascalculated by the above-described expression (13).

As a result, the reaction rate was 72% in the case of (i), 88% in thecase of (ii), 98% in the case of (iii), and 83% in the case of (iv).

It is to be noted that unreacted NaNO₃ remained in the reaction productin the case of (i). Furthermore, in the case of (iv), the proportion ofNaNO₃ to be volatilized before reaction with the WC-based cementedcarbide scraps was increased. Accordingly, the reaction rate wasdecreased.

Example 25

By dilution, with air, of the exhaust gas containing NO gas generated bythe reaction between WC—TiC-based cemented carbide and the molten saltof NaNO₃, the NO concentration was set at 3200 ppm. Then, in accordancewith the reaction represented by the above-described expression (3), theabove-described exhaust gas was decomposed into nitrogen and water inthe presence of an oxide catalyst of titanium at a temperature of 200°C. while changing the mixing ratio of ammonia gas.

Consequently, when mixing ammonia gas of 1.02 equivalent with respect toNO gas, the NO concentration in the exhaust gas could be decreased to0.5 ppm though some unreacted ammonia gas was still detected.

Furthermore, when mixing ammonia gas of 0.99 equivalent with respect toNO gas, the NO concentration in the exhaust gas could be decreased to 40ppm while unreacted ammonia gas was not detected.

Furthermore, when mixing ammonia gas of 0.96 equivalent with respect toNO gas, the NO concentration in the exhaust gas could be decreased to150 ppm while unreacted ammonia gas was not detected.

Furthermore, when mixing ammonia gas of 0.93 equivalent with respect toNO gas, the NO concentration in the exhaust gas could be decreased to210 ppm while unreacted ammonia gas was not detected.

It was confirmed that the NO gas in the exhaust gas without containingammonia gas could be set at a low concentration of 150 ppm or less sincethe equivalent amount of ammonia gas was approximately 0.96 to 1.00 thatcould be controlled for the reaction of the NO gas amount withstability.

Furthermore, it was found from X-ray diffraction that the reactionproduct produced by the reaction between the molten salt of NaNO₃ andCo, Ni, TiC, and TaC which are components other than WC contained in theabove-described WC—TiC-based cemented carbide is an oxoacid sodium saltsuch as NaCO₂O₄, NaNiO₂, Na₈Ti₅O₁₄, and NaTaO₃.

Then, water was caused to act on each oxoacid sodium salt describedabove, which was subsequently neutralized with an acid to achieve a pHof 7 or lower, for undergoing hydrolysis. Consequently, it was foundfrom X-ray diffraction that the above-described oxoacid sodium saltswere changed to water-insoluble CO₃O₄, NiO, TiO₂, Na₃TaO₄, and Ta₂O₅,respectively.

Example 26

The reaction product was obtained by causing a reaction between themolten salt of NaNO₃ and WC—TiC-based cemented carbide obtained bypeeling off the thin film of the coated chip coated with a thin filmsuch as TiC, TiN, TiCN, or Al₂O₃.

Then, the reaction product obtained as described above was dissolved in200 ml of water to produce a precipitate which was then removed byfiltration. Then, hydrochloric acid was added to the filtrate obtainedby filtration, which was then neutralized to pH 7.

The component of the above-described filtrate neutralized by addinghydrochloric acid was examined by ICP atomic emission spectroscopy, andthe component of the precipitate produced by adding hydrochloric acid tothe filtrate was also examined by an X-ray microanalyzer.

As a result, 6.24 atomic percent of W, 0.0002 atomic percent of Co, notmore than 0.0001 atomic percent of Ti (the quantitative lower limit ofICP atomic emission spectroscopy), 0.0008 atomic percent of Ta, 0.0005atomic percent of Nb, and not more than 0.0001 atomic percent of Fe weredetected from the filtrate neutralized by adding hydrochloric acid.

Furthermore, Co, Ti, Ta, and O were detected from the precipitate. Itwas confirmed that the component of the precipitate was an oxide of Co,an oxide of Ti, or an oxide of Ta which was insoluble in water.

The results of Examples as described above show that the presentinvention allows tungsten to be separated and collected as sodiumtungstate from the tungsten containing material.

Example 27

An apparatus having a series of structures shown in FIG. 1 wasmanufactured, in which reaction container 17 was made of stainless steeland had a capacity of 410 liters. The WC—TiC-based cemented carbideobtained by peeling off a thin film from the chip coated with a thinfilm such as TiC, TiN, TiCN, or Al₂O₃ was supplied through supply port1, and NaNO₃ was supplied through sodium nitrate supply port 2, to causea reaction on the conditions equivalent to those of Example 1 inTable 1. Consequently, the supplied cemented carbide could be dissolvedwithout causing overflow.

The reaction product that was placed in container 18 and cooled wasexamined by X-ray diffraction. As a result, only a single phase ofNa₂WO₄ was recognized. It was also confirmed that the pH of the aqueoussolution obtained by dissolving 3 g of product in 200 ml of water was11.68 and the reaction occurred as in the expression (1). In X-raydiffraction, unreacted NaNO₃ was not detected in the reaction product.In addition, the compound of cobalt and titanium has relatively weakdiffraction intensity as compared with that of Na₂WO₄, and no clear peakwas recognized.

Furthermore, the gas produced by the reaction was removed throughnitrogen oxide outlet port 16 and directed to the apparatus forrendering nitrogen oxide harmless (expression (3)). Consequently, thenitrogen oxide concentration in the processed exhaust gas showed a lowvalue of 0 to 17 ppm. Thus, it could be confirmed that the nitrogenoxide could be rendered harmless.

Examples 28 to 35 and Comparative Example 1

First, the WC—Co-based cemented carbide having chromium and vanadiumadded thereto was reacted with the molten salt of NaNO₃, to produce amelt-like reaction product, which was then maintained at 720° C.

Then, the above-described reaction product was distributed tocontainers. With respect to the mass 1.00 of the above-describedreaction product, tungsten (W) powder having a ratio of 0.29 in Example28, tungsten carbide (WC) powder having a ratio of 0.29 in Example 29,carbide powder (main component: WC—Co) having a ratio of 0.29 in Example30, carbide powder (main component: WC—Co) having a ratio of 0.44 inExample 31, carbon (C) powder having a ratio of 0.29 in Example 32,carbon (C) powder having a ratio of 0.05 in Example 33, tungsten (W)powder having a ratio of 0.45 in Example 34, and carbide powder (maincomponent: WC—Co) having a ratio of 0.54 in Example 35 each as areductant were introduced into separate containers, and held and reactedfor 4 hours at the temperatures shown in Table 1, and then compared withComparative Example 1 to which no reductant was added.

Then, the melt-like product having the above-described reductantintroduced thereinto was cooled and solidified. Then, 3 grams ofproducts were obtained from each container and dissolved in 200 ml ofpure water, which was then filtered to produce an aqueous solution ineach of Examples 28 to 35 and Comparative Example 1.

Then, the pH of the aqueous solution in each of Examples 28 to 35 andComparative Example 1 was evaluated. In addition, the content of each ofchromium (Cr) and vanadium (V) was also analyzed by ICP atomic emissionspectroscopy. The results are shown in Table 2.

TABLE 2 Reductant Processing Aqueous Solution Addi- Tem- Cr V tionperature Content Content Type Ratio (° C.) pH (g/L) (g/L) Example 28 WPowder 0.29 850 9.53 0.0003 0.0087 Example 29 WC Powder 0.29 720 9.610.0005 0.0103 Example 30 Carbide 0.29 720 9.88 0.0002 0.0050 PowderExample 31 Carbide 0.44 720 7.42 0.0001 0.0007 Powder Example 32 CPowder 0.29 720 11.37 0.0008 0.0187 Example 33 C Powder 0.05 700 11.490.0191 0.0195 Example 34 W Powder 0.45 800 7.31 0.0003 0.0008 Example 35Carbide 0.54 720 7.24 0.0001 0.0006 Powder Comparative — — 700 11.600.0570 0.0196 Example 1

As shown in Table 2, it was confirmed that the content of Cr in theaqueous solution in each of Examples 28 to 35 into which W powder, WCpowder, carbide powder, and C powder as reductants were introduced wasremarkably low with respect to the amount of Cr dissolved in thereaction product in the aqueous solution (0.057 g per liter) ofComparative Example 1 into which the above-described reductant was notintroduced. This shows that Cr was rendered insoluble in water byintroduction of the above-described reductant.

The results shown in Table 2 indicated that the reducing power for Crwas shown in the order of carbide powder>W powder>WC powder>C powder. Inaddition, it was confirmed that little Cr remained in the aqueoussolution when the reductant that was introduced in the above descriptionwas used.

As shown in Table 2, it was confirmed that the content of V in theaqueous solution in each of Examples 28 to 35 into which W powder, WCpowder, carbide powder, and C powder as reductants were introduced wasremarkably low with respect to the amount of V dissolved in the reactionproduct in the aqueous solution (0.0196 g per liter) of ComparativeExample 1 into which the above-described reductant was not introduced.This shows that V was rendered insoluble in water by introduction of theabove-described reductant.

The results shown in Table 2 indicated that the reducing power for V wasshown in the order of carbide powder>W powder>WC powder>C powder. Inaddition, when the C powder among the reductants that were introduced inthe above description was used as a reductant, the reduced amount of thecontent of V in the aqueous solution was decreased. Thus, it isconsidered that the reducing action of V is relatively small and awater-soluble vanadium compound still exists even when V is reduced.

The reaction between WC—Co-based cemented carbide and the molten salt ofNaNO₃ proceeded in accordance with the above-described expression (1).In addition, the produced sodium oxide was reacted with water andconverted into sodium hydroxide. Accordingly, as shown in Table 2, theaqueous solution in Comparative Example 1 into which no reductant wasintroduced was strongly alkaline.

On the other hand, as shown in Examples 28 to 35, the pH of the aqueoussolution is generally lowered in accordance with an increase in theamount of the introduced reductant and was neutral in each of Examples31, 34 and 35. Accordingly, it is considered that sodium oxide in theaqueous solution was almost completely converted into sodium tungstate.

In addition, it was confirmed that, when C powder was introduced as areductant, sodium oxide hardly reacted and the pH was not so lowered, asshown in Examples 32 and 33.

Furthermore, the precipitate obtained by filtration during production ofthe aqueous solution in each of the above-described Examples 28 to 35was dried and examined by an EDX analysis, with the result that Co wasdetected from all of the precipitates. This indicates that Co isrendered insoluble in water in the aqueous solution of each of Examples28 to 35. In Example 35, unreacted W powder was found also in theprecipitate since W powder as a reductant was excessively introduced.

Example 36

First, 1 kg of hard scraps of WC—Co-based cemented carbide (WC-basedcemented carbide) was introduced into the reaction container made ofstainless steel. The hard scraps contained in the reaction container washeated to 720° C. using a heating furnace, to which 1.2 kg of NaNO₃powder was then supplied for 3 hours, to cause a reaction for producinga reaction product. In addition, the temperature of the heating furnacewas adjusted to keep the temperature of the reaction product to fallwithin the range of 720° C. to 750° C. for keeping it as a melt untilthe reaction was completed.

Then, in the state where the temperature of the melt of the reactionproduct was kept at 720° C., soft scraps of WC-Co-based cemented carbide(WC-based cemented carbide) were supplied by each constant amount for 2hours to the melt of the reaction product.

Consequently, it was confirmed that the amount of the soft scrap to besupplied is adjusted to cause the reaction between the above-describedhard scraps and the molten salt of NaNO₃ to be gently developed, suchthat the reaction can be controlled. Furthermore, it was also confirmedthat soft scraps are supplied to the melt of the above-describedreaction product to cause a reaction of sodium oxide which is aby-product, with the result that sodium tungstate is obtained.

The products produced during the reaction developed by supplying thesoft scraps to the melt of the reaction products were sampled more thanonce in accordance with development of the reaction. The sampledproducts each were then dissolved in water to produce an aqueoussolution. Then, the concentration of each of Cr and V dissolved in theaqueous solution was analyzed by ICP atomic emission spectroscopy. Theresults thereof are shown in FIG. 9. In FIG. 9, the vertical axis showsthe concentration of each of Cr and V in the aqueous solution, and thehorizontal axis shows the ratio of the amount of the introduced softscraps with respect to the mass of the melt of the reaction product.

As shown in FIG. 9, approximately 100% of Cr could be rendered insolublein water by reduction caused by introduction of soft scraps.Furthermore, while merely approximately 60% of V could be renderedinsoluble in water by reduction caused by introduction of soft scraps,it could still be confirmed that V could be rendered insoluble in waterby introduction of the reductant.

In addition, when the reaction product obtained by the reaction betweenthe above-described hard scraps and the molten salt of NaNO₃ wasdissolved, as is, in pure water to produce a Na₂WO₄ aqueous solution,sodium oxide which is a by-product of the above-described reaction isreacted with water to produce sodium hydroxide. Accordingly, the aqueoussolution was strongly alkaline which was around pH 11.

On the other hand, the above-described soft scraps were introduced intothe reaction product obtained by the reaction between theabove-described hard scraps and the molten salt of NaNO₃, which was thendissolved in pure water to produce a Na₂WO₄ aqueous solution. In thiscase, sodium oxide which is a by-product of the above-described reactionwas consumed by introduction of the above-described soft scraps.Consequently, the aqueous solution was slightly alkaline which was equalto or less than pH 8. Therefore, it is considered also preferable that,in the case where soft scraps are introduced, the amount of acid to beintroduced can be reduced even when acid is introduced forneutralization in the subsequent process.

Example 37

After producing the melt of the reaction product by the reaction betweenthe hard scraps of WC-Co-based cemented carbide (WC-based cementedcarbide) and the molten salt of NaNO₃ in the reaction container, WCpowder was introduced as soft scraps in the mass ratio of 0.30 withrespect to a mass 1.00 of the melt of the reaction product.

Then, after the above-described melt of the reaction product was cooledand solidified, it was dissolved in warm water of 80° C. to produce anaqueous solution. In this case, about 30% of the solidified reactionproduct was not dissolved in water but remained in the state of arock-like solid matter and adhered to the reaction container. Then, thecomponent of the solid matter remained adhered to the reaction containerwas analyzed by the X-ray diffraction method. FIG. 10 shows an X-raydiffraction pattern of the above-described solid matter obtained by theX-ray diffraction method. In FIG. 10, the vertical axis shows X-raydiffraction intensity (cps), and the horizontal axis shows a diffractionangle 2θ(°).

In the X-ray diffraction pattern in FIG. 10, X-ray diffraction peakscorresponding to Na₂WO₄, WC and Co, respectively, were recognized. TheX-ray diffraction peak corresponding to WC indicates that the WC powderintroduced as described above was not completely reacted with sodiumoxide and remained.

In addition, it is considered that oxygen is required to cause areaction between the WC powder and sodium oxide, as represented in theabove-described expression (10). Furthermore, another example of themethod for causing the reaction between the WC powder and sodium oxidemay include a method in which WO₃ powder of the oxide is introducedtogether with WC powder as a reductant in order to cause the reactionrepresented in the above-described expression (11).

After producing the melt of the reaction product by the reaction betweenthe hard scraps of WC—Co-based cemented carbide (WC-based cementedcarbide) and the molten salt of NaNO₃ in the reaction container, themixture of WO₃ powder and WC powder that is a main component of softscraps was introduced in the mass ratio of 0.35 with respect to a mass 1of the melt of the reaction product. It is to be noted that the molarratio of the above-described mixture was WC:WO₃=1:4.

After the above-described melt of the reaction product was cooled andsolidified, it was dissolved in warm water of 80° C. to produce anaqueous solution, with the result that the solidified reaction productcould be completely dissolved in contrast to the case where WC powderalone was introduced.

Then, the above-described aqueous solution was filtered to therebyobtain insoluble residue from the above-described aqueous solution. Thecomponent of the residue was then analyzed by the X-ray diffractionmethod. FIG. 11 shows an X-ray diffraction pattern of theabove-described residue obtained by the X-ray diffraction method. InFIG. 11, the vertical axis shows X-ray diffraction intensity (cps), andthe horizontal axis shows a diffraction angle 2θ(°).

While only a slight X-ray diffraction peak corresponding to WC wasrecognized in the X-ray diffraction pattern in FIG. 11, it was confirmedthat most WC was consumed by the reaction with sodium oxide.Furthermore, any X-ray diffraction peak corresponding to Na₂WO₄ was notrecognized in the X-ray diffraction pattern in FIG. 11. Accordingly, itwas also confirmed that produced Na₂WO₄ did not remain in the aqueoussolution but was entirely extracted by water.

Therefore, it was found that it is effective to supply oxygen in orderto remove sodium oxide by adding WC powder and soft scraps to thereaction product produced by the reaction between the above-describedhard scraps and the molten salt of NaNO₃.

In addition, an example of the method for supplying oxygen may include amethod in which WO₃ is used together with WC as a reductant as describedabove.

It should be understood that the embodiments and examples disclosedherein are illustrative and non-restrictive in every respect. The scopeof the present invention is defined by the terms of the claims, ratherthan the description above, and is intended to include any modificationswithin the scope and meaning equivalent to the terms of the claims.

Industrial Applicability

The present invention can be applied to a method for producing sodiumtungstate, a method for collecting tungsten using the method and anapparatus for producing sodium tungstate.

The present invention can be applied to a method for producing a sodiumtungstate aqueous solution and a method for collecting tungsten.

Description of the Reference Signs

1 tungsten containing material supply port, 1 a tungsten containingmaterial introduction pipe, 1 b, 2 b storage unit, 2 sodium nitratesupply port, 2 a sodium nitrate introduction pipe, 3 tungsten containingmaterial, 4 sodium nitrate, 5 protection container, 6 heating device, 7rotation shaft, 8 holding container, 9 melt, 10 outer wall portion, 11discharge port, 12 roller, 13 protection container, 14 heating device,15 inlet port, 16 outlet port, 17 reaction container, 18 container, 21trough, 22 dust-collecting outlet port, 23 automatic supply device, 25soft scrap, 27, 37 stirrer, 28 gas introduction port, 29 melt dischargepassage, 31 powder supplier, 39 water, 41 holding furnace, 42 waterdissolution bath.

The invention claimed is:
 1. A method for producing sodium tungstatefrom a tungsten containing material, said method comprising the stepsof: continuously or intermittently supplying at least one of saidtungsten containing material and an oxidant made of one of sodiumnitrate and sodium nitrite; heating at least one of said oxidant andsaid tungsten containing material; and continuously producing a reactionproduct by bringing said tungsten containing material and said oxidantinto contact with each other in an atmosphere containing oxygen, in thestep of continuously producing said reaction product, said tungstencontaining material is contained in a holding container and at least apart of said tungsten containing material contained in said holdingcontainer is immersed in a melt containing at least one of said oxidantand said reaction product, said holding container having a side providedwith a plurality of openings and a bottom provided with a communicationhole, said openings and said communication hole each being sized so asto prevent passage of said tungsten containing material, said holdingcontainer being immersed in said melt such that at least half of saidtungsten containing material is immersed in said melt.
 2. The method forproducing sodium tungstate according to claim 1, wherein a heatingtemperature in said step of heating is 700° C. or more and 850° C. orless.
 3. The method for producing sodium tungstate according to claim 1,wherein at least a part of said reaction product is removed from acontact interface between said tungsten containing material and saidoxidant.
 4. The method for producing sodium tungstate according to claim1, wherein a total amount of said oxidant to be supplied is 1.2 times ormore and 4 times or less of a total mass of said tungsten containingmaterial.
 5. The method for producing sodium tungstate according toclaim 1, wherein an amount of said tungsten containing material immersedin said melt is kept constant.
 6. The method for producing sodiumtungstate according to claim 1, wherein at least a part of nitrogenoxide contained in an exhaust gas generated together with said reactionproduct is decomposed to set a concentration of said nitrogen oxide insaid exhaust gas to be continuously at 150 ppm or less.
 7. The methodfor producing sodium tungstate according to claim 1, comprising thesteps of: hydrolyzing an oxoacid sodium salt produced by dissolving saidreaction product in water; and filtering out a water-insoluble matterproduced by said hydrolyzing step to extract sodium tungstate.
 8. Amethod for collecting tungsten, comprising the method for producingsodium tungstate according to claim 1.